#298440 by Moonshine and KarenS: move form permission checking to content_field_form() and don't call hook_widget for users w/out permission.

Userref / Noderef : Add a 'none' choice for non-required, single fields with 'Radios / checkboxes' widget.

- minor : 'synchronize' the code between userref and noderef.

Reverted a few commits that I'd prefer to keep in -dev for now

(the 'validate' op is still supported, though)

Display field type human names instead of machine names on 'Manage Fields' and 'Fields' overview pages.